Bruce D. Trudgill

Fault Growth by Segment Linkage - An Explanation for Scatter in Maximum Displacement and Trace Length Data from the Canyonlands Grabens of SE Utah

Maximum displacement (D) and trace length (L) data for a population of 97 normal faults from the Canyonlands Grabens region of SE Utah are presented. Values of L range from 100 to 6500 m, and of D from 1.5–155 m. The data exhibit a scatter between D and L of about half an order-of-magnitude. This is comparable to that exhibited in other published single population datasets. This magnitude of scatter cannot be attributed either to measurement errors or to variation in mechanical properties. We propose that a scatter of this magnitude can be explained by a general model for fault growth by segment linkage, whereby large incremental increases of length attained as fault segments link temporarily exceed incremental increases in displacement. This results in deviations from an idealized growth path, and a step-like evolution expressed between D and L.

Relay-ramp forms and normal-fault linkages, Canyonlands National Park, Utah

Fieldwork within a series of mesoscale grabens in southeast Utah has revealed a particularly well-exposed system of interlinked extensional faults. A series of down-faulted grabens are developed within a 460-m-thick brittle layer of upper Paleozoic sandstone and shale, which overlies a ductile layer with a high gypsum content. All the major grabens consist of two or more overlapping elements, which are composed of fault segments. These segments may be hard-linked (fault surfaces are joined) or soft-linked (fault surfaces are isolated, but linked by ductile strain of the rock volume between them) in map view. Relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments. In the Canyonlands grabens, the transfer of displacement between soft-linked fault segments is characterized by well-de-fined, dipping relay ramps, commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments. Relay ramps develop as ephemeral structures, eventually becoming breached by hard linkage of the fault segments. Breakdown of ramps by breaching is part of the process of fault growth by segment linkage. Within the Canyonlands graben system, four orders of segmentation are present. This may be a consequence of different scales of mechanical heterogeneity, but evidence from the Canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage.

Salt-Related Fault Families and Fault Welds in the Northern Gulf of Mexico

Salt-related faults and fault welds in the northern Gulf of Mexico are classified based on the three-dimensional geometry of the faults or welds, deformed strata, and associated salt. Kinematic or genetic criteria are not used in the classification. Only documented fault styles are considered; those styles produced by experimental or numerical modeling, but not yet observed in the Gulf, are not included. Extensional faults comprising symmetric arrays include peripheral faults, which occur at the landward margin of the original salt basin; crestal faults, which are growth faults rooted in reactive diapirs; and keystone faults, which occur at the crests of anticlines. Asymmetric arrays of normal faults are grouped according to the dominant dip direction. Faults that dip primarily basinward include roller faults, which are listric growth faults that sole into a subhorizontal salt layer; ramp faults, which extend upward from the landward margin of bulb-shaped salt stocks; and shale-detachment faults, which sole into a shale decollement that merges into a salt layer. Counterregional faults are landward-dipping asymmetric arrays that link cylindrical, basinward-leaning salt stocks. Asymmetric arrays with variable dip direction include flap faults, whose footwalls comprise diapirs with uplifted and rotated roof strata, and rollover faults, which occur at the hinges of monoclinal folds. Two families of contractional faults are described: toe thrusts, which are basinward-vergent thrusts that ramp up from a salt or shale decollement, and break thrusts, which are high-angle reverse faults that cut one or both limbs of detachment folds. Fault arrays that strike parallel to the regional dip direction are termed lateral faults. Six types of fault welds are defined: primary welds are those at the autochthonous level; roho welds are subhorizontal, allochthonous welds into which roller faults detach; counterregional welds comprise both subhorizontal and landward-dipping segments beneath growth monoclines; bowl welds are elliptical and upwardly concave; thrust welds are landward-dipping surfaces that separate repeated stratigraphic sections; and wrench welds are steep and strike parallel to the regional dip direction. Groups of geometrically classified fault families and fault welds are kinematically and genetically linked to each other and to associated salt bodies and welds. Linked fault systems can contain extensional, contractional, and strike-slip components. Extensional fault families are formed by basinward translation, subsidence into salt, or folding. Those fault families that accommodate basinward translation are balanced by salt extrusion or contractional fault families. Strike-slip fault families commonly provide hard links, although various fault components also can be soft linked. We illustrate five associations of linked fault systems that are directly related to five types of salt systems: autochthonous salt, stepped counterregional, roho, salt-stock canopy, and salt nappe.

The Perdido fold belt, northwestern deep Gulf of Mexico; Part 1, Structural geometry, evolution and regional implications

The Perdido fold belt is a frontier petroleum exploration province located in deep waters of the northwestern Gulf of Mexico. The anticlines are northeast-southwest-trending, symmetrical to asymmetrical, with concentric folds usually bounded on both flanks by steep reverse faults. The folds are interpreted as detachment folds cored by autochthonous Middle Jurassic Louann Salt. The fold belt overlies rifted transitional crust characterized by northeast-southwest-trending basement highs and northwest-southeast transverse structures that controlled the original salt thickness and subsequent fold geometry. Upper Jurassic-Eocene strata were folded during the early Oligocene (36-30 Ma), with deformation possibly continuing into the earliest Miocene. Postkinematic sediments gradually buried the folds, with younger strata progressively onlapping the highest structures. Some folds were reactivated during the middle Miocene, and a late phase of broad uplift during the Pliocene-Pleistocene is attributed to loading of the Louann Salt by the advancing Sigsbee salt nappe. The Perdido fold belt marks the basinward margin of a complex, linked system of gravitational spreading above salt. Updip Paleogene sedimentary loading and associated extension were accommodated downdip primarily by salt canopy extrusion. The 5-10% shortening and folding occurred only after canopy feeders were evacuated and closed off. Subsequent loading and deformation were concentrated at higher, allochthonous levels.

The growth of normal faults by segment linkage

Abstract Fault growth is widely described using a scaling law between maximum displacement (D) and length (L), of the form D = cLn. This expression defines a model of fault growth by radial propagation from a single seed fracture or fault. This paper presents geometrical and kinematic evidence from a set of exceptionally well exposed normal faults in Utah for an alternative model of fault growth. This model is referred to as growth by segment linkage, and involves the propagation and linkage of independent fault segments on ascending length scales. The evidence presented focuses on the geometry and displacement variation in the region of relay structures, and on local scaling relationships between D and L. The D - L data from 97 faults in the study area range over three orders of magnitude, and show a general trend to increasing D for increasing L. There is a large scatter in the data, similar to that recognized in previous D - L compilations. It is argued that the scatter cannot be attributed either to measurement errors or to variation in mechanical properties. Instead, we argue that the model of growth by segment linkage provides a simple explanation of this scatter, and propose that the process of segment linkage may explain scatter in other datasets.

Structural Controls on Drainage Development in the Canyonlands Grabens of Southeast Utah

The Canyonlands grabens in southeast Utah form an active exten sional fault array covering 200 km2 southeast of the Colorado River. The fault array formed as a result of gravity gliding above a thick layer of salt. Growth of this fault array within the last 0.5 m.y. (possibly last 0.1 m.y.) has resulted in major changes in the stream drainages across the area through processes of stream capture and diversion. During growth of the fault array, relay ramps between overlapping fault segments form topographic lows along the graben margins. These commonly act as access points for captured streams to enter a graben system. As fault segments continue to propagate laterally, linkage leads to breaching of the relay ramp structures. This causes changes in the course and gradients of the streams, com monly shifting the locus of alluvial sediment deposition away from grabens that were previously infilling. This complex evolution of drainage networks in a growing fault array may provide a valuable analog to the early structural and stratigraphic development of larger continental rift systems. Reservoir distribution is an essential element of many hydrocarbon plays, and understanding the rela tionship between active fault growth and drainage evolution may help predict reservoir distribution and quality.

The Perdido Fold Belt, Northwestern Deep Gulf of Mexico, Part 2: Seismic Stratigraphy and Petroleum Systems

Analysis of 12,000 km of two-dimensional multifold seismic data shows a thick succession of Mesozoic and Cenozoic deep-water strata in the Perdido fold belt, northwestern deep Gulf of Mexico. These strata differ in seismic facies, areal distribution, and reservoir/petroleum potential. Mesozoic strata are interpreted as dominantly fine-grained carbonates and show minor thickness changes. Cenozoic strata are largely mud-dominated siliciclastic turbidite deposits and vary considerably in thickness across the fold belt. These changes reflect the shifting position of Cenozoic marginal-marine depocenters. Mesozoic reservoir potential consists of fractured Upper Jurassic and Cretaceous deep-water carbonates. Cenozoic reservoir potential consists of siliciclastic deep-water turbidites. Portions of the Paleocene to lower Eocene strata are sand-prone and are the downdip equivalents of the lower and upper Wilcox shallow-marine depocenters. These strata are all incorporated within the folds. Lower to middle Oligocene strata coincide with the main growth phase of the fold belt. Potentially sand-prone middle Oligocene to lower Miocene strata are the downdip equivalents of the Vicksburg (early Oligocene), Frio (Oligocene), and Oakville (early Miocene) shallow-water depocenters. These strata form potential stratigraphic traps against the folds. Mesozoic source potential was modeled assuming Oxfordian, Tithonian, Barremian, and Turonian source beds. One-dimensional thermal maturation modeling showed these sources reached peak oil generation between 51 and 39 Ma, 39 and 8 Ma, 32 and 2 Ma, and 26 and 8 Ma, respectively. Cenozoic source potential was modeled using an Eocene source. Modeling showed this source reached only early oil generation in the basinward half of the fold belt. Thermal maturation was reached by source beds at different times in different locations due to changes in burial depth, amount of structural uplift, and underlying thickness of autochthonous salt. All of these factors indicate that seal and reservoir carry significant risk, but that the potential exists for large petroleum accumulations.

Three-dimensional geometry and evolution of a salt-related growth-fault array: Eugene Island 330 field, offshore Louisiana, Gulf of Mexico

Abstract The geometry and evolution of a growth-fault array bounding the Eugene Island 330 Field were studied using 3-D seismic interpretation and analysis of displacement patterns. The 3-D geometry of the kinematically coherent fault array is complex and characterized by both lateral and vertical branching and linkage of individual fault segments, so that fault patterns and interactions very considerably between different structural levels. Three types of faults are distinguished: growth faults with throw increasing downward; compensation faults with closed tip lines; and composite faults, which combine elements of both growth and compensation faults. Reconstruction of the fault arrays evolution using displacement backstripping illustrates the growth and linkage of some isolated faults and the upward separation of others into distinct segments. The results may have important implications for understanding the history of hydrocarbon migration and entrapment in the EI-330 field and other fault-bounded minibasins in the Gulf of Mexico.

Colorado and Adjacent Areas

This first volume in the series includes trips held in conjunction with the 1999 Annual Meeting in Denver, Colorado, with more than 12 GSA-sponsored field trip guides presented in one volume. This year, the GSA Field Guide exposes readers to the beauty and diversity of the Colorado region, including trips focusing on sedimentology, hydrogeology, coal areas, tectonics, and other disciplines.

Integrating 3D Seismic Data with Structural Restorations to Elucidate the Evolution of a Stepped Counter-Regional Salt System, Eastern Louisiana Shelf, Northern Gulf of Mexico

Abstract By integrating 3D and 2D seismic interpretation with structural restorations we have reconstructed the evolution of a complex, composite stepped counter-regional salt system in the West Delta/South Pass (WDSP) area of the northern Gulf of Mexico. Biostratigraphically calibrated well data allow the last 10 Ma of the evolution of the salt system to be divided into six stages: (1) sea-floor extrusion of isolated salt tongues fed from the Jurassic Louann salt through northward dipping feeders prior to 7.5 Ma; (2) amalgamation of the salt tongues to form a salt-tongue canopy between 7.5 and 6.4 Ma; (3) counter-regional evacuation of the salt-tongue canopy as a result of enhanced sediment loading due to progradation of the shelf margin between 6.4 and 5.0Ma; (4) evacuation of salt into a series of salt walls linking salt domes between 5.00 and 2.55 Ma; (5) evacuation of the salt walls to form counter-regional fault welds between 1.95 and 0.5 Ma; and (6) final evacuation of most of the salt from deeper levels leaving a series of isolated salt domes connected by counter-regional fault welds. The counter-regional evacuation of the WDSP salt systems illustrates the value and limitations of published 2D models for allochthonous salt, and the reconstructed evolution yields insights into the complex interactions between salt deformation and sedimentation. The results also suggest that the WDSP salt systems significantly affected sediment transport pathways, trap geometries and possibly late stage petroleum migration across evacuating salt welds. Supplementary material: Three movies depicting the 2D and 3D restorations and models are available at https://doi.org/10.6084/m9.figshare.c.4784595